Precious metal adhesion to glass is involved in a number of industrial and aesthetic applications. Optical and infrared reflectors and lenses can be made to reflect and/or transmit different wavelengths depending upon the particular precious metal film applied. Often such films are also applied in patterns utilizing their ability to conduct electricity. Finally, precious metals, especially gold, have many decorative uses in glass and crystal objects.
U.S. Pat. No. 3,266,912, assigned to Engelhard Corporation, describes a family of products known as "liquid bright golds". These are varnish-like precious metal preparations containing organic compounds combined with flux components which are based on organic metal compounds. The flux components act as an adhesion promoter. On a smooth glass ceramic surface, a high gloss metal film forms after firing in a high temperature oven or annealer. This film has a gold or other precious metal content of 6 to 15%. Such compounds are used in a number of aesthetic and industrial applications in a relatively straightforward application of precious metal to glass. However, the products have some disadvantages. The organic compounds and flux agents essential to such solutions are environmentally toxic. Moreover, the presence of such agents in the fired film or coating compromise the aesthetic and physical properties of such films. The fluxes discolor when applied to certain types of glasses. This causes the color of the precious metal coating to have two tones; depending upon whether one is viewing the film from the surface of the vitreous substrate or through the actual substance the film is applied to. Furthermore, the liquid bright films have a distinct plated look as the gold is formed on the top and not absorbed or melted into the surface of the vitreous substances. From a material efficiency viewpoint, the relatively low precious metal content of such films degrades the electrical, thermal, and optical properties of the films.
U.S. Pat. No. 4,837,052 describes a technology which involves sandwiching a layer of deposited gold between an inner coating of gold-chromium and an outer protective polymer overcoat. In this process, adhesion to the vitreous substrate is achieved through the application first of gold-chromium. Pure gold is then applied to the layer of gold coating but is very complex, requiring three applications. Moreover, successful adhesion is only achieved through the application of a polymer foreign substance.
A third approach to achieving precious metal adhesion is; described in U.S. Pat. No. 2,950,996. This technology involves; combining gold resinates and vitreous substances together in one compound. This mixture is applied to the vitreous surface. Upon firing, the mixture forms a combination gold/vitreous compound which adheres to the vitreous surface as well as to the gold particles suspended in the composition. This technique is effective for many technical applications but also has shortcomings. Particularly, the film is never 100% gold. Moreover, only gold resinates may be used, these compounds limiting the appearance and physical characteristics of the adhered precious metal. Finally, the vitreous substances must have low melting points relative to the glasses they are coated upon, in order that the contour of the coated glass does not distort. Currently, such low melting glasses contain lead. This prevents the use of such coatings in functional glassware.